The Science and Engineering of Sound

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Sound Wave – A vibrational disturbance that involves mechanical motion of molecules transmitting energy from one place to another.

Compression – Push of air molecules creating a peak of high pressure.

  • Reducing a signal’s output level in relation to its input level to reduce dynamic range.

Rarefaction – A trough of low pressure.

Longitudinal Wave – The repetition of compression and rarefaction creates a longitudinal wave or sound wave.

Wave Form – Changes in air pressure created by the sound wave against time.

Wavelength – Distance between two peaks of a wave.

Velocity – Speed in a given direction.

Amplitude – The magnitude of a sound wave or an electric signal, measured in decibel’s.

Frequency – The # of times per second that a sound source vibrates, expressed in hertz (Hz).

Hertz – Unit of measurement of frequency; numerically equal to cycles per second (cps).

440 Hertz – The standard musicians use to tune instruments, which corresponds to the A note on second space treble clef.

Phase – Factor in the interaction of one wave with another, either acoustically or electronically.

  • It also references, in a multiple microphone recording, the place a microphone samples the sound wave in relation to another microphone sampling the same sound wave.

Fundamental  Frequency – The lowest frequency a sound source can produce.

  • In other words, it is also called the first harmonic or primary frequency which is the lowest, or basic, pitch of a musical instrument. an example is 440 Hertz.

Harmonic – Is a multiple of the fundamental frequency,  For example, 880 Hertz is the first  harmonic of 440 Hertz.

Octave – The interval between two frequencies that have a tonal ratio of 2:1. For example, 880 Hertz is one octave above 440 Hertz.

Envelope – Describes the shape of a sound over time.

Attack – The time it takes for a sound to build up to full volume.

Decay – How quickly a sound levels off to a sustain.

Sustain – The ongoing sound.

Release – How quickly the sound decays or dissipates after the note is released.

Infrasonic – The range below the frequencies audible to human hearing.

Ultrasonic – The range above the frequencies of human hearing.

Pitch – The subjective perception of frequency – the highness or lowness of a sound.

Sound Frequency Spectrum – The range of frequencies audible to human hearing: about 20 to 20,000 Hz.

Bass – The low range of the audible frequency spectrum; usually from 20 to 320 Hz.

Midrange – The part of the frequency spectrum to which humans are most sensitive; the frequencies between roughly 320 Hz and 2,560 Hz.

Treble – Frequency range between roughly 5,120 Hz and 20,000 Hz, the highest two octaves audible to human hearing in the sound frequency spectrum.

Equalization – A signal-processing device that can boost, attenuate, or shelve frequencies in a sound source or sound system.

Decibel (dB) – A relative and dimensionless unit to measure the ratio of two quantities.

  • Decibels are not a static unit measurement, it’s a comparison measurement and you always need a reference.
  • Soft whisper around 30 dB SPL to freight train at 100 ft from 70dB SPL up to a jet takeoff from 200 feet registering at 120 dB SPL.
  • Safety regulations – 85 dB and below you will be fine for 8 hours.
    • Cut that time in half each 3 dB you go up.
  • Every 3 dB higher doubles the signal strength.